The present invention relates to a field effect transistor and a method for fabricating the same, and more particularly, it relates to a high-power transistor, a high-frequency transistor or the like using a nitride semiconductor and a method for fabricating the same.
A group III-V nitride compound semiconductor typified by gallium nitride (GaN) or the like, namely, what is called a nitride semiconductor, is a wide gap semiconductor, and the band gap at room temperature of, for example, gallium nitride (GaN) is 3.4 eV and that of aluminum nitride (AlN) is 6.2 eV. A nitride semiconductor is characterized by having a large breakdown electric field and having higher electron saturated drift velocity than a compound semiconductor such as gallium arsenic (GaAs) or silicon (Si) semiconductor. Therefore, it is regarded as a promising material for a high-frequency and high-power transistor and is earnestly being studied and developed.
In a hetero junction structure of aluminum gallium nitride (AlGaN) and gallium nitride (GaN), charges are produced on the hetero junction interface on the (0001) plane through spontaneous polarization and piezo polarization. Owing to the charges produced on the hetero junction interface, a sheet carrier concentration of 1×1013 cm−2 or more can be attained even when it is undoped. By utilizing a two-dimensional electron gas generated on the hetero junction interface, a hetero junction electric field transistor with a high current density can be realized. Accordingly, a field effect transistor using a nitride semiconductor is advantageous for attaining high output.
In general, for improving the high-frequency characteristics of a field effect transistor, it is the most effective means to reduce the gate length. For example, in order to improve the maximum oscillation frequency fmax, it is necessary to increase mutual conductance gm corresponding to a gain, to reduce the capacitance around the gate electrode and to reduce the resistance of the gate electrode.
For reducing the gate length in using a conventional gallium arsenic (GaAs)-based or indium phosphorus (InP)-based compound semiconductor, a structure and process for forming a T-shaped or mushroom-shaped gate electrode have been proposed and put to practical use. Also, in using a GaN-based semiconductor, a T-shaped gate electrode or the like has been studied and improvement of the high frequency characteristic of the maximum oscillation frequency fmax attained by such a gate electrode has been reported.
In using a GaN-based semiconductor, parasitic resistance such as ohmic contact resistance of a source electrode and a drain electrode tends to be large. Therefore, in order to realize a field effect transistor with good high frequency characteristics by effectively using high saturated drift velocity, it is necessary to increase the maximum electric field formed below the gate electrode as compared with the case where a GaAs-based compound semiconductor is used. Accordingly, it is difficult to realize a device with high frequency characteristics equivalent to or better than those of the GaAs-based or InP-based semiconductor unless a shorter gate length than in using a conventional compound semiconductor is realized.
FIG. 9 shows the cross-sectional structure of a conventional short-gate length field effect transistor using a nitride semiconductor such as GaN. As shown in FIG. 9, an undoped GaN layer 803 and an n-type AlGaN layer 804 are successively formed above a sapphire substrate 801 with a low-temperature GaN buffer layer 802 sandwiched between the sapphire substrate 801 and the undoped GaN layer 803. A source electrode 805 and a drain electrode 806 made of a multilayer of titanium (Ti) and aluminum (Al) are formed on the n-type AlGaN layer 804 to be spaced from each other. A T-shaped gate electrode 807 made of a multilayer of nickel (Ni), platinum (Pt) and gold (Au) is formed between the source electrode 805 and the drain electrode 806.
A gate length corresponding to the width of a contact portion between the T-shaped gate electrode 807 and the n-type AlGaN layer 804 is approximately 150 nm. The T-shaped gate electrode 807 may be formed through, for example, formation of a three-layered resist structure, electron-beam deposition and lift-off. For example, as the lowermost and the uppermost layers of the three-layered resist structure that can be exposed with electron beams, a resist of polymethyl methacrylate (PMMA) or the like is used, and as the intermediate layer, a resist of polydimethyl glutaric imide (PMGI) or the like is used. After exposing the uppermost PMMA resist by a width of approximately 1 μm, an opening is formed in the intermediate PMGI resist by using a developer. At this point, a peak portion of the T shape is formed in the uppermost PMMA resist. Subsequently, the lowermost PMMA resist exposed on the bottom of the opening is exposed with electron beams by a width of 150 nm.
The GaN-based electric field transistor including the T-shaped gate electrode has large mutual conductance, small capacitance around the gate electrode and low gate resistance, and hence can realize good high-frequency characteristics (see, for example, Y. F. Wu et al., “International Electron Devices Technical Meeting”, 2003, pp. 579-581).
In the procedure for forming the T-shaped gate electrode of the conventional field effect transistor using the nitride semiconductor, however, it is necessary to perform the electron-beam exposure. Also, it is necessary to use the three-layered photoresist, and therefore, the cost of the procedure for forming the gate electrode is disadvantageously high. Furthermore, reproducibility of the procedure for forming the T-shaped gate electrode is poor, and the gate electrode easily collapses, which disadvantageously lowers the yield.